The present invention relates to a system for controlling an expansion/contraction operation of a cylinder for vertically moving a driven member, such as a hoist cylinder of a vehicle particularly for vertically swinging a vessel of a dump truck or an arm cylinder for vertically swinging an arm of an excavation loader.
In the dump truck, a hoist cylinder is arranged between a vehicle body (truck body) and a vessel so that the vessel is swung upward through the expansion motion of the hoist cylinder to take a standing state and swung downward through the contraction motion of the hoist cylinder to take a tilt-down state in which the vessel is settled on the vehicle body.
In such dump truck, when the contraction motion of the hoist cylinder is made fast, the vessel is swung downward in an increased speed so that the vessel in the standing state can be tilted i.e., descending, down in a short time. However, if the vessel is swung downward rapidly, the vessel is seated or settled on the vehicle body with a large shock (called, seating shock, hereinlater).
Contrary to the above, when the contraction motion of the hoist cylinder is made slow, the seating shock is reduced. However, it takes much time for the vessel now in the standing state to take its tilt-down position.
In the prior art, in order to obviate such defects as mentioned above, there have been proposed various techniques for reducing the seating shock by making slow the contraction speed of the hoist cylinder just before the seating (settling) on the vehicle body, i.e., stroke-end time.
For example, in a device disclosed in Japanese Patent Laid-open Publication No. HEI 9-119405, at a time when the movement of the hoist cylinder approaches its stroke end position (i.e., a position just before the vessel seating time on the vehicle body), a hoist valve is displaced to a snub position and a return oil from an expansion chamber of the hoist cylinder passes through an orifice of the hoist valve disposed to the snub position. According to such arrangement, the flow rate of the oil is restricted to thereby make slow the contraction speed of the hoist cylinder.
In another example of Japanese Patent Laid-open Publication No. 2000-87916, there is shown a device for reducing a shock at a cylinder stroke end position, in which there is arranged a cylinder for reducing a shock at a stroke end of the cylinder by throttling a return oil after the approaching to the contraction stroke end. In the use of such cylinder, an electromagnetic proportional pressure reducing valve is incorporated in a circuit connecting a directional control valve and a remote control valve so as to detect a signal representing an increasing of a load pressure due to the throttling of the return oil of the cylinder and, upon the detection of such signal, the electromagnetic proportional pressure reducing valve is current-conducted and controlled to thereby displace the directional control valve to a position near a neutral position to more reduce the shock at the cylinder stroke end position.
In the former prior art device, the return oil of the hoist cylinder passes through the orifice of the hoist valve displaced to the snub position, so that an amount of the oil passing through the orifice varies under influence of difference of viscosities at various oil temperatures. For this reason, the temperature of the return oil of the hoist valve differs at a time when an atmospheric temperature differs or a temperature at a working site differs, or at a time when a working condition differs such as a difference between at an early morning working time and at a time after long working period. Accordingly, in such time, the contraction speed of the hoist cylinder differs, and in such case, there may cause a case that the seating shock cannot be reduced.
In the latter prior art device, on the other hand, it becomes necessary to use a cylinder having a complicated structure for throttling the return oil after the approaching to the stroke end position, so that an expensive cylinder is required and an entire device becomes also expensive.
An object of the present invention is therefore to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a system for controlling an operation of a cylinder capable of reducing a shock to a driven member such as vessel in a dump truck at its descending stroke ends.
This and other objects can be achieved according to the present invention by providing a system for controlling an operation of a cylinder comprising:
a cylinder displacing a member to be driven in a vertical direction;
a directional control valve being switched so as to take a first position performing an expansion motion of the cylinder and a second position performing a contraction motion of the cylinder;
a throttling means for throttling a meter-out area at a time when the directional control valve takes the second position;
means for outputting a signal in response to the vertical displacement of the driven member; and
a controller,
the controller being provided with a function for detecting a vertical position and a descending speed of the driven member in accordance with the signal from the signal outputting means, a function for controlling the throttling means at a time just before the driven member reaches a descending end position so as to make meter-out area to an optimum throttling value at which a descending shock is small, and function for detecting a descending speed of the driven member in a time interval from the position just before the descending end to the descending end position and amending and controlling the meter-out area by controlling the throttling means in accordance with the detected descending speed so as to make coincident the descending speed with the set descending speed.
According to the structure of the above aspect, when the driven member is descended at a position just before the descending end, the meter-out area of the directional control valve is throttled and the flow rate of the return oil from the cylinder is reduced to thereby make slow the contraction speed of the cylinder.
When the driven member is further descended from the position just before the descending end, the aforementioned meter-out area is amended and controlled in accordance with the descending speed and then descended to the descending end position at the set descending speed.
Therefore, in a case where the descending speed of the driven member becomes fast or slow according to the difference in the viscosity of the oil in different temperatures, the meter-out area of the directional control valve changes, and hence, the driven member is descended to the descending end position at the set descending speed.
Accordingly, the shock of the driven member at the descending end position can be reduced in an occasion of different working temperatures such as in the presence of an atmospheric temperature difference or temperature difference in the working sites, or in an occasion of different working or operating conditions such as in an early morning operation time or in a long time working.
Furthermore, according to the present invention, since the vertical position and the descending speed of the driven member can be detected in accordance with the output signal from the signal outputting means, a cheap cylinder of usual type can be utilized, and hence, the entire device can be manufactured with reduced cost.
Still furthermore, since the meter-out area is controlled at a portion between the position just before the descending end and the descending end position, the controlling time can be converged within a constant range, so that there is less influenced by differences in the meter-out area due to the weight of the driven member or the working error of the directional control valve, in comparison with the case where the vertical movement of the driven member is controlled over the entire stroke range, thus surely reducing the shock at the descending end position of the driven member.
Furthermore, in the structure of the present invention mentioned above, the controller is further provided with a learning function for amending an optimum throttling valve of a next meter-out area to be small at a time when the descending time of the driven member from the position just before the descending end to the descending end position is longer than the optimum time, and on the contrary, for amending the optimum throttling valve of the next meter-out area to be large at a time ho when the descending time of the driven member from the position just before the descending end to the descending end position is shorter than the optimum time.
According to this structure, at a time when the descending time of the driven member from the position just before the descending end to the descending end position is different from the optimum time (i.e., the driven member is not descended at the set descending speed), the next optimum meter-out area is changed in accordance with the actual descending time, so that the driven member can be descended at the optimum time in the next descending operation.
Furthermore, in the present invention, the directional control valve is a pilot pressure change-over valve to be changed over by a pilot pressure, the throttling means is an electromagnetic proportional pressure control valve for controlling the pilot pressure, and the controller controls the meter-out area of the directional control valve by controlling a current conducted to a solenoid of the electromagnetic proportional pressure control valve.
According to this structure, the expansion and contraction operations of the driven member can be controlled by using the directional control valve of a usual pilot pressure change-over valve type and the electromagnetic proportional pressure control valve. Moreover, at this time, the controller merely controls the current conduction to the solenoid, so that this controlling can be easily done.
The nature and further characteristic features of the present invention will be made more clear from the following descriptions made with reference to the accompanying drawings.